The closest prior art known are U.S. Pat. No. 2,781,343; 02/12/57 to K. W. Swain for CURRENT-LIMITING FUSE COMPRISING FUSE LINKS OF SILVER AND COPPER, and U.S. Pat. No. 2,809,257 to K. W. Swain for COMPOSITE FUSE LINKS OF SILVER AND COPPER. These two patents solve the problem of reducing the fusing and vaporization I.sup.2 .multidot.t value of the fusible element in terms of (Amps.multidot./cm.sup.2).sup.2 .multidot.sec. without resorting to a fusible element which is all of silver. The present invention provides a fusible element having a greater current-carrying capacity than the fusible elements disclosed in the above referred-to patents to Swain on account of more effective cooling means than those disclosed by Swain.
According to the present invention, the composite fusible element comprises a strip of silver having a predetermined thickness and a strip of copper having a thickness in excess of said predetermined thickness. The strip of copper has a groove and the strip of silver is placed into said groove and forms an inlay in said strip of copper. In other words, the strip of copper is wrapped around the strip of silver, such greatly increasing the interface between both metals in comparison to Swain.
As in Swain, a pair of juxtaposed incisions sever entirely said strip of copper and reduce the cross-section of said strip of silver at the point where said pair of incisions is located.
An interface is, however, formed where said strip of silver and said strip of copper meet. The cross-section of said interface has a length larger than twice the thickness of said strip of silver. Both in Swain and in applicant's structure the copper parts of the fusible element form cooling fins for cooling a strip of silver. The inlay of a strip of silver in a base of copper results, however, in a much larger interface between silver and copper than in Swain because in Swain the area of the two interfaces between silver and copper is roughly only equal to the narrow edges where the two metals meet, while in the structure according to the present invention the interface between silver and copper also includes the relatively wide bottom surface of the strip of silver and the relatively large bottom surface of the groove in the strip of copper.
Thus there will be a large heat-flow from the silver insert to the copper base, and from there to the pulverulent arc-extinguishing filler normally surrounding the copper base.